1. Field of the Invention
The present invention relates generally to methods and apparatus for manufacturing honeycomb structures. More particularly, the present invention relates to an improved method and apparatus for manufacturing thermally fused honeycomb structures.
2. Description of Related Art
Honeycomb core materials are widely used in industry due to their light weight and unique structural characteristics. Honeycomb cores made from relatively thin gauge thermoplastic sheets are especially useful because they are lightweight and provide flexible structures which can withstand high compression loads. Such honeycomb structures are commonly found in applications as diverse as aircraft components and running shoes.
There are a variety of ways to manufacture honeycomb cores. One common method of manufacture is typified by U.S. Pat. No. 3,660,217, issued to Kehr et al. This method entails the formation of lines of adhesive onto sheets of material arranged in a stack. The line pattern formed on each individual sheet is staggered in relation to the previous sheet. Once the adhesive lines are all in place, the stack is compressed and heated, if necessary, until the adhesive has fully cured. The resulting structure is commonly referred to as a honeycomb before expansion or stack.
In order to form the final honeycomb structure, the stack is expanded by applying a uniform outward force to the outermost sheets of the stack. Heat is usually applied during the expansion process in order to permanently set the thermoplastic in its final expanded honeycomb shape.
The use of adhesives to bond the honeycomb layers together is relatively simple and is acceptable for many applications. However, a variety of factors may contribute to uneven bonding which often yields less than desirable results. For example, when using heat-cured adhesives, which typically have a lower melting point than the melting point of the sheet material, the outermost sheets of the stack may receive an excess amount of heat, while the innermost sheets receive too little heat. As a consequence, uneven bonding throughout the structure may result. It is evident then, that the dissimilar adhesive material is usually the weak link in the structure, and when the stack is expanded, the weaker bond areas may come apart under the pressure of expansion and cause gaps or tears in the honeycomb structure. The present invention eliminates the weak link associated with adhesive bonding of the sheets by providing a fused bond zone formed directly in the adjoining honeycomb layers which exhibits properties substantially identical to the properties of the honeycomb material.
Another technique for manufacturing honeycomb structures involves using weld bonds in place of adhesives. In general, this procedure involves forming the stack by sequentially welding one layer of thermoplastic material to the stack at a time. It is essential that the top sheet which is being welded to the underlying stack of layers be welded only to the uppermost layer of the stack. Inadvertent welding of the underlying layers prohibits the expansion necessary to form the honeycomb structure.
In order to prevent inadvertent welding of the underlying layers, various protective inserts are placed directly beneath the weld location. These protective inserts are typically referred to as "fingers" and are designed to prevent the top two layers which are being welded together from sticking to the remainder of the stack. U.S. Pat. No. 4,957,577 issued to Huebner, discloses an exemplary procedure which utilizes protective fingers.
The protective fingers work well in welding processes to prevent bonding of the uppermost layers to the body of the stack. However, use of the protective fingers is undesirable in that they must be inserted underneath each weld line. This is a time consuming process and, if a finger is inadvertently left out, the two uppermost layers are welded to the underlying stack. The result is a partially blocked honeycomb structure. In addition, the fingers must be removed once the stack is expanded. This can present problems, especially when the lateral dimensions of the honeycomb are large.
As is apparent from the above, it would be desirable to eliminate the need for protective fingers when welding the thermoplastic layers together while still providing adequate weld bonds.